Nattaporn Chattham

Optically driven translational and rotational motions of microrod particles in a nematic liquid crystal

Significance This paper addresses light-driven dynamic motions of microrods suspended in a liquid crystal (LC) host. The results presented in this paper advance molecular-assisted manipulations of colloidal particles and their assemblies, which is a key concept for smart microdevices, using their interplay with the topology of the nematic director field. Almost all previous studies on colloids in LC (except some work by Yamamoto et al.) have dealt with the observation and analysis of defect structures around colloid particle(s) with fixed anchoring conditions. Herein we use a photoactive surfactant, which enables control of the LC anchoring by light irradiation. The induced reversible anchoring transition is accompanied by a reorientation of the rods and/or their translational motion. A small amount of azo-dendrimer molecules dissolved in a liquid crystal enables translational and rotational motions of microrods in a liquid crystal matrix under unpolarized UV light irradiation. This motion is initiated by a light-induced trans-to-cis conformational change of the dendrimer adsorbed at the rod surface and the associated director reorientation. The bending direction of the cis conformers is not random but is selectively chosen due to the curved local director field in the vicinity of the dendrimer-coated surface. Different types of director distortions occur around the rods, depending on their orientations with respect to the nematic director field. This leads to different types of motions driven by the torques exerted on the particles by the director reorientations.

Optical manipulation of the nematic director field around microspheres covered with an azo-dendrimer monolayer.

We report here the optical manipulation of the director and topological defect structures of nematic liquid crystals around a silica microparticle with azobenzene-containing dendrimers (azo-dendrimers) on its surface. We successfully demonstrate the successive switching processes from hedgehog, to boojum, and further to Saturn ring configurations by ultraviolet (UV) light irradiation and termination. The switching time between these defect structures depends on the UV light intensity and attains about 50 ms. Since the pretreatment of microparticles is not necessary and the surface modification is spontaneously performed just by dissolving the azo-dendrimers in liquid crystals, this dendrimer supplies us with a variety of possible applications.

Direct observation of two-dimensional nematic and smectic ordering in freely suspended films of a bolaamphiphilic liquid crystal

The T-shaped bolaamphiphile CT2, a liquid crystal with a hydroxyl-terminated, rod-shaped core and semifluorinated side chains, forms the Lam-Iso, Lam-Nem, and Lam-Sm lamellar phases, which are reported to exhibit respectively smectic layering of the side chains with additional in-layer isotropic (Iso), nematic (Nem) and smectic-like (Sm) ordering of the rigid molecular cores. Optical observation of 2–16 layer thick freely suspended films enables visualization and quantitative study of these transitions in two dimensions (2D). The 2D-Iso to 2D-Nem transition is found to be first order, occurring first in the film surface layers and then in the film interior on further cooling. The second order 2D-Nem to 2D-Sm transition is marked by the continuous evolution of the orientational texture from 2D nematic-like to 2D focal conic. The film geometry stabilizes the lamellar ordering, significantly increasing the birefringence associated with the in-plane ordering in the films.

Precession Mechanism of Nematic Liquid Crystal Droplets under Low Power Optical Tweezers

Optical tweezers is a magnificent tool for microscopic manipulation. Owing to few piconewton noninvasive trapping force, the tiny objects from micro sand beads down to living bacteria can be trapped under optical tweezers system. Liquid crystals (LC) are materials that enriched of optical properties providing various phemomena that can be applied for technology. In this report, we applied optical tweezers to nematic liquid crystal system for the optical manipulation study. 5CB (4-cyano-4’-pentylbiphenyl) was used with an appropriate surfactant to prepare nematic liquid crystal (NLC) droplets at room temperature. NLC droplets in radial and bipolar configurations can be formed. They react with light angular momentum and reveal dynamic and static changes through spinning and changing of internal configuration. Several recent articles reported spinning dynamics of NLC droplets under high power trapping (more than a few hundred milliwatts), however, high power caused disturbance in internal configuration of droplets. Dynamic changes of droplet under low power trapping (lower than 100 mW) conducting so far were reported unsuccessful. Here we report the first investigation on behaviour of radial NLC droplet under low power optical trap. The configuration of droplets was not disturbed under low power trapping. We investigated the dynamic behaviour of non-disturbed NLC droplet in low power optical trap.

Controlled rotation of lipid tubules with optical tweezers

Chiral Phospholipids are found self-assembled into fascinating cylindrical tubules of 500 nm in diameter by helical winding of bilayer stripes under cooling in ethanol and water solution. Theoretical prediction and experimental evidence reported so far confirmed the modulated tilt direction in a helical striped pattern of the tubules. This molecular orientation morphology results in optically birefringent tubules. We manipulated birefringent lipid tubules under 532 nm linearly polarized laser tweezers. Spontaneous rotation of lipid tubules induced by radiation torque was observed with only one sense of rotation caused by chirality of lipid tubules. Rotation discontinues once the high index axis of lipid tubule aligned with a polarization axis of the laser. Thus, by controlling the direction of linearly polarized light, angle of tubule rotation can be specified. This observation holds promising applications in nano- and bio-technologies.

Azodendrimers as a photoactive interface for liquid crystals

ABSTRACT Dendrimers with azo moieties in their peripherals (azodendrimers), which we developed, are useful as a photoactive interface for liquid crystal (LC) systems. The azodendrimer spontaneously adsorbs on a variety of interfaces, so that we can obtain photoactive surfaces without pretreatment only by introducing a small amount of the azodendrimers in LCs. We first describe our previous studies applied for a variety of LC systems such as sandwich cells, LC droplets and shells in a solvent and microparticles in LCs. Some unpublished results on nematic droplets of bent-shaped molecules demonstrate current developments in the research of the interactions between the photoswitchable dendrimer and complex liquid crystalline structures. Graphical Abstract

Application of optical tweezers and excimer laser to study protoplast fusion

Protoplast fusion is a physical phenomenon that two protoplasts come in contact and fuse together. Doing so, it is possible to combine specific genes from one protoplast to another during fusion such as drought resistance and disease resistance. There are a few possible methods to induce protoplast fusion, for example, electrofusion and chemical fusion. In this study, chemical fusion was performed with laser applied as an external force to enhance rate of fusion and observed under a microscope. Optical tweezers (1064 nm with 100X objective N.A. 1.3) and excimer laser (308 nm LMU-40X-UVB objective) were set with a Nikon Ti-U inverted microscope. Samples were prepared by soaking in hypertonic solution in order to induce cell plasmolysis. Elodea Canadensis and Allium cepa plasmolysed leaves were cut and observed under microscope. Concentration of solution was varied to induce difference turgor pressures on protoplasts pushing at cell wall. Free protoplasts in solution were trapped by optical tweezers to study the effect of Polyethylene glycol (PEG) solution. PEG was diluted by Ca+ solution during the process to induced protoplast cell contact and fusion. Possibility of protoplast fusion by excimer laser was investigated and found possible. Here we report a novel tool for plant cell fusion using excimer laser. Plant growth after cell fusion is currently conducted.

Microsurgery of Elodea Cells Using Excimer Laser

Gene transfer is a genetic engineering technique aiming to insert an interested gene into a specific host organism. Since its time of discovery, many gene transfer methods have been successfully evolved for practical use in the laboratory. However, non of them are ideal methods for all organism. Each method is only suitable to particular gene and particular host organism due to their different physical characteristics. Especially, plant cells with thick cell wall are one of the most difficult samples for gene transfer because of their high tolerance to external stimulation. Here, we present the combination between laser technology and microscopy to enhance the quality of plant gene transfer by applying the knowledge of protoplast technology. Elodea cells were used as sample cells and were treated in solution for plasmolysis. Excimer laser and Optical tweezers system were set for microsurgery of Elodea sample cells and insertion of DNA filled liposome into targeted cell. The ongoing process of protoplast and liposome fusion is under investigation.

Precession mechanism of nematic liquid crystal droplet under optical tweezers

Optical Tweezers are well known for manipulating and tracking microscopic particles used in many biological and microfluidic applications. Trapping birefringent particles, e.g. liquid crystal droplets, gives insight into the aspect of light polarization in optical tweezers. The outstanding properties of liquid crystal droplets are their high refractive index and birefringent property suitable for light angular momentum transfer. Under the microscope, the Maltese cross of radial nematic liquid crystal droplet was observed. Trapped under 1064 nm Optical Tweezers with power lower than 80 mW, the droplet precession around the focal point of the laser beam was observed due to circular polarization of laser. In this study we show that the precession behavior of radial nematic droplet depends on the degree of ellipticity of polarization state of light, power of the laser source and size of the radial nematic droplet, affecting the induced electrical polarization and internal reordering of the droplets. The theoretical explanation and the model of this behavior have also been determined and discussed.

Rate of growth pattern of yeast cells studied under optical tweezers

Cell growth and division has been of scientists’ interest for over generations. Several mathematical models have been reported derived from conventional method of cell culture. Here we applied optical tweezers to guide cell division directionally. The patterns of Saccharonmyces bayanus yeast growth was studied under 1064 nm line optical tweezers generated by time-shared multiple optical traps. Yeast growth was found following the path of the generated laser patterns in linear, circular, square and L shapes, speculatively as a result of localized heating effect due to absorption at the focal point.